Light-transmitting metal electrode having hyperfine structure and process for preparation thereof

ABSTRACT

The present invention provides a metal electrode transparent to light. The metal electrode comprises a transparent substrate and a metal electrode layer composed of a metal part and plural openings. The metal electrode layer continues without breaks, and 90% or more of the metal part continues linearly without breaks by the openings in a straight length of not more than ⅓ of the visible wavelength to use in 380 nm to 780 nm. The openings have an average diameter in the range of not less than 10 nm and not more than ⅓ of the wavelength of incident light, and the pitches between the centers of the openings are not less than the average diameter and not more than ½ of the wavelength of incident light. The metal electrode layer has a thickness in the range of not less than 10 nm and not more than 200 nm.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of, and claims the benefitof priority under 35 U.S.C. §120 from, U.S. application Ser. No.12/187,653, filed Aug. 7, 2008, which claims the benefit of priorityunder 35 U.S.C. §119 from Japanese Patent Applications No. 2007-245167,filed on Sep. 21, 2007. The entire contents of each of the aboveapplications are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a light-transmitting metal electrode.In detail, the invention relates to a light-transmission metal electrodehaving a hyperfine structure. The invention also relates to a processfor preparation of the light-transmitting metal electrode.

2. Background Art

Light-transmitting electrodes, which have light transparencyparticularly in the visible region and at the same time which functionas electrodes, are widely used in electronics industries. For example,all the displays distributed currently in markets, except displays ofcathode ray tube (CRT) type, need light-transmitting electrodes sincethey adopt electric driving systems. According as flat panel displaystypically such as liquid crystal displays and plasma displays have beenexplosively getting popular in recent years, the demand forlight-transmitting electrodes has been rapidly increasing.

In early studies of electrodes that transmit light, the electrodes weremainly made of a metal such as Au, Ag, Pt, Cu, Rh, Pd or Cr in the formof such very thin foil having a thickness of 3 to 15 nm that the metalfoil could have light transparency to a certain degree. When used, forexample, the thin metal foil was put between transparent dielectriclayers for improving durability. However, since the foil was made of ametal, there was a trade-off relationship between resistivity andlight-transmittance and hence it could not have properties satisfyingenough to put various devices into practical use. The mainstream study,therefore, shifted to oxide semiconductors. In present, almost all thepractical light-transmitting metal electrodes are made of oxidesemiconductor materials. For example, indium tin oxide (hereinafter,referred to as “ITO”), which is indium oxide containing tin as a dopant,is generally used.

However, as described below in detail, the trade-off relationshipbetween resistivity and light-transmittance is essentially still presenteven in oxide semiconductor materials. The problem in metal foil is thatthe light-transmittance decreases in accordance with increase of thefoil thickness, while the problem in oxide semiconductor materials isthat the light-transmittance decreases in accordance with increase ofthe carrier density. Accordingly, the problem to study is only changedfrom the former to the latter.

As described above, the demand for light-transmitting metal electrodesis expected to keep expanding in the future in many applications, butthere are some future problems.

First, there is a fear that indium, which is employed as a material forthe electrodes, will be exhausted. Indium is a major component of ITO,which is widely used in the light-transmitting electrodes, and is henceexpected to be exhausted in the worldwide range according as the demandfor displays typically such as flat panel displays increases rapidly. Itis a real fact that there is a shortage of rare metals such as indium,and accordingly the cost of materials has really risen remarkably. Thus,this is a serious problem.

To cope with this problem, for example, in the sputtering process forforming an ITO film, it is studied to reuse even an ITO membranedeposited on the inner wall of vacuum chamber so as to improve theefficiency of ITO target to the utmost limit. However, techniques likethat only postpone the exhaustion of indium and they by no meansessentially solve the problem. In consideration of that, indium-freelight-transmitting electrodes are currently being developed. However, atpresent, any substitute such as zinc oxide material or tin oxidematerial is not yet capable of exhibiting properties exceeding ITO.

The second problem is that, if the carrier density is increased toimprove electro conductivity of oxide semiconductor material, thereflection in a longer wavelength region is increased to lower thetransmittance. The reason for this is as follows.

According to electronic states, substances are classified into twotypes: some substances have energy gaps, and the others do not. Evenwhen the substances having energy gaps are irradiated with light havingenergy smaller than the gaps, they do not absorb the light becauseelectrons do not undergo the band transition. Therefore, with respect tovisible light in the wavelength region of 380 nm to 780 nm, thesubstances having energy gaps of more than 3.3 eV are transparent to thelight.

On the other hand, depending on the width of the energy gap between thevalence band and the conduction band, substances are generallycategorized into three types, namely, conductors, semiconductors andinsulators. The substances having relatively small band gaps areconductors, and in contrast those having relatively large band gaps areinsulators, and those having middle band gaps are semiconductors. Oxidesemiconductors, which are assigned to semiconductors, have chemicalbonds of strong ionic character and hence generally have large energygaps. Accordingly, they can readily satisfy the above condition at ashorter wavelength in the visible region, but the transparency at alonger wavelength is liable to lower. Further, in the case where theoxide semiconductors are used in light-transmitting electrodes, carriersof electron drift, namely, carriers of electric current are doped toobtain conductivity and transparency to visible light. For example, ITOconsists of In₂O₃ containing SnO₂ as a dopant. In this way, oxidesemiconductors can be made to have low resistivities by increasing thecarrier densities. However, according as the carrier density isincreased, the electrode layer of oxide semiconductor as a whole becomesexhibiting metallic behavior and consequently the transmittance becomesdecreasing from at a longer wavelength. If the transmittance in avisible region becomes lower, the electrode can not work aslight-transmitting electrode well. Because of this phenomenon, there isa lower limit to the resistivity of light-transmission electrode made ofoxide semiconductor.

In view of this, there are attempts to lower the resistivity oflight-transmitting metal electrode. For example, a transparent substrateis coated with a metal mesh electrode having a thickness of not morethan 15 μm, a line width of not more than 25 μm and an opening diameterof 50 μm to 2.5 mm. The openings of the mesh are then filled with atransparent resin film. On the composition thus prepared, an ITO film isprovided to produce a light-transmitting electrode (see,JP-A-2005-332705 (KOKAI), for example). However, even in this electrode,the metal mesh electrode serves as only an assistant to the ITO film andhence does not solve the above problems.

Because of the aforementioned circumstances, it is desired to provide alight-transmitting metal electrode made of a conductive material whichis versatile and inexpensive, which is free from the fear of exhaustionand at the same time which can keep a low resistivity, namely, a highconductivity.

SUMMARY OF THE INVENTION

According to the present invention, there is provided alight-transmitting metal electrode comprising:

a transparent substrate, and

a metal electrode layer having a thickness in the range of not less than10 nm and not more than 200 nm formed on said substrate, comprising

a metal part that any pair of point-positions in the part iscontinuously connected, and

a plurality of openings penetrating through said metal electrode layerhaving an average diameter in the range of not less than 10 nm and notmore than ⅓ of said wavelength of light, the pitches between the centersof said openings being not less than the average diameter and not morethan ½ of said wavelength of light,wherein 90% or more of said metal part in said metal electrode layercontinues linearly without breaks by the openings in a straight lengthof not more than ⅓ of the wavelength of visible light to use in 380 nmto 780 nm.

According to the present invention, there is also provided a firstprocess for preparation of the above electrode, wherein

a phase-separated block-copolymer membrane is formed in the form of aclot pattern of microdomains, and then an etching process is carried outby using said dot pattern of microdomains as a mask to prepare a metalelectrode layer comprising openings.

According to the present invention, there is also provided a secondprocess for preparation of the above electrode, comprising the steps of

preparing a transparent substrate,

forming an organic polymer layer on said transparent substrate,

forming an inorganic substance layer on said organic polymer layer,

forming a phase-separated block-copolymer membrane in the form of a dotpattern of microdomains on said inorganic substance layer,

transferring said dot pattern of block-copolymer microdomains onto theorganic polymer layer and said inorganic substance layer, so thatcolumnar structures made of said organic polymer and said inorganicsubstance are formed on the surface of said transparent substrate,

forming a metal layer onto the columnar structures, and

removing said organic polymer.

Still further, according to the present invention, there is provided adisplaying device comprising the above light-transmitting metalelectrode.

The present invention provides a light-transmitting metal electrodehaving high transparency while keeping a low resistivity by using ametal as the electroconductive material for the electrode. Since thehigh transparency of the electrode is given by the particular hyperfinestructure, the metal used as the material can be selected from a widerange metals. This means that it is unnecessary to use conventional raremetal oxide materials, and accordingly a versatile and inexpensivelight-transmitting metal electrode can be provided. Further, it is alsopossible to make a breakthrough into the lower limit to resistivities oflight-transmitting electrodes made of conventional oxide semiconductors.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an example of a pattern of the light-transmissionmetal electrode comprising openings.

FIG. 2 is an electron micrograph showing an example of a pattern of thelight-transmission metal electrode comprising openings according to oneembodiment of the present invention.

FIG. 3 illustrates an example of the process for preparation of thelight-transmission metal electrode comprising openings according to oneembodiment of the present invention.

FIG. 4 is an electron micrograph showing an example of a pattern of thelight-transmission metal electrode comprising openings according toanother embodiment of the present invention.

FIG. 5 is a chart showing a transmittance, in the visible region, of thelight-transmission metal electrode comprising openings according to oneembodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

First, with respect to the response of substance irradiated with light,the basic theory is explained below.

Drude theory, which describes movement of free electrons from theviewpoint of classical mechanics, tells us as follows. On the assumptionthat the mean scattering time of free electrons is much shorter than theperiod of oscillation of light, the dielectric function ∈(ω) isexpressed by the following formula (1):∈(ω)=∈_(b)(ω)−ωp ²/ω²  (1)whereinωp=(ne²/m×∈₀)^(1/2) is a plasma frequency of conduction electrons, andhere n is a carrier density, e is an electric charge, m is an effectivemass, and ∈₀ is the permittivity of vacuum. The first term of theformula (1) stands for contribution of the metal dipoles, and is approx.1, here. The second term stands for contribution of the conductionelectrons.

As shown above, the plasma frequency is a function of the carrierdensity n. In the case of ω₀>ω, the dielectric function ∈(ω) gives anegative value and hence the incident light undergoes plasma reflection.On the other hand, it the case of ω>ω₀, the dielectric function ∈(ω)gives a positive value and hence the light transmits through thesubstance. Accordingly, the plasma frequency can be regarded as thethreshold between reflection and transmittance when the substanceresponds to light.

A typical metal has a plasma frequency in the ultraviolet region, andhence reflects visible light. For example, silver Ag has a carrierdensity of approx. n=6.9×10²² (cm⁻³), and the wavelength correspondingto the plasma frequency is positioned at approx. 130 nm in theultraviolet region.

In contrast, the wavelength corresponding to the plasma frequency of theoxide semiconductor ITO is positioned in the infrared wavelength region.The carrier density is in direct proportion to the electroconductivityand is in inverse proportion to the resistivity. This means that, if thedopant is increased to lower the resistivity, the plasma frequency isincreased. Accordingly, when the dopant is increased to reach aparticular amount, visible light in a longer wavelength region undergoesthe plasma reflection to lower the transmittance.

As described above, in order that the oxide semiconductor material canensure transparency in the visible region, the wavelength correspondingto the plasma frequency must be positioned in the infrared region. Thecarrier density, therefore, has an upper limit. For this reason,generally produced ITO has a carrier density of approx. n==0.1×10²²(cm⁻³), which is several percents of those of metals. The lower limit ofresistivity derived from the above value is approx. 100 μΩ·cm, and it isessentially impossible to reduce the resistivity more.

Thus, the light-transmitting electrode of oxide semiconductor like ITOhas a theoretical lower limit to the resistivity. However, as theelectronics technology is developed, particularly, as mobile instrumentsequipped with displays such as cellular phones and notebook-size PCs aredeveloped, it is evidently getting required to reduce the resistivity oflight-transmitting electrode so as to prevent the electric powerconsumption from increasing. In spite of that, it is difficult toovercome the above trade-off relationship by only the conventionaltechnology.

In consideration of the aforementioned problem, the present invention isthought out.

The light-transmitting metal electrode and the process for preparationthereof according to the present invention are explained below in detailwith the attached drawings referred to.

First, the theoretical bases of the present invention are describedbelow.

FIG. 1 shows one embodiment of the light-transmitting metal electrodeaccording to the present invention. FIG. 1(A) is a sketch of thelight-transmitting metal electrode, and FIG. 1(B) is an elevationthereof. The light-transmitting metal electrode comprises a smooth andtransparent substrate 1 and a metal electrode layer 2 provided thereon.The metal electrode layer 2 comprises a metal part 3 and fine openings 4penetrating through the metal part. The metal electrode layer 2 canfunction as an electrode and at the same time can work aslight-transmitting layer in the visible wavelength region.

The light-transmission metal electrode according to the presentinvention is characterized in that the transparency is more thanexpected from the total area occupied by the openings 4 in the metalelectrode layer, and in other words, is characterized in that thereflection essentially ascribed to the metal part is fundamentallyreduced to transmit light.

The above metal electrode layer has openings much smaller than thewavelength of light incident on the electrode, and thereby can serve asa light-transmitting electrode although made of a metal. There areroughly two reasons for this. One is that, when the electrode is exposedto light, movement of free electrons induced by the electric field ofthe light is inhibited since the metal part continues linearly withoutbreaks by the openings in a straight length of not more than ⅓ of thewavelength of light. Consequently, the electrode is transparent to thelight. The other is that, since the openings have diameters much smallerthan the wavelength of light, effects of Rayleigh scattering anddiffraction are reduced to keep straightforwardness of the light.

Here, the term “wavelength of light” means a wavelength of lightincident on the light-transmission electrode when the electrode is used.Accordingly, the wavelength can be selected in a wide range, but is inthe visible wavelength region of 380 nm to 780 nm (see, for example,“KAGAKU JITEN (dictionary of chemistry, written in Japanese)”, publishedby Tokyo Kagaku Dozin Co., Ltd.). In order to ensure a satisfyingtransmittance of the electrode, the transparent substrate has atransmittance of preferably 80% or more, more preferably 90% or more.The above term “straight length” means the maximum straight distancebetween two point-positions not separated by the openings in any area onthe electrode surface.

The first theoretical basis is then described below. The Drude theorydescribed above is based on the assumption that the object substance ishomogeneous and sufficiently large as compared with the wavelength ofincident light. When light having a frequency lower than the plasmafrequency is applied to the substance, free electrons in the substanceare polarized by the electric field of the light. The polarization isinduced in such direction that the electric field of light may becancelled. The electric field of light is thus shielded by the inducedpolarization of electrons, so that the light does not penetrate into thesubstance and thus, what is called, “plasma reflection” is observed.

If the substance in which electrons are induced to be polarized issufficiently small as compared with the wavelength of light, it isthought that the movement of electrons is restricted by the geometricalstructure and, as a result, that the electric field of light cannot beshielded.

As described above, in the present invention, the response of substanceto light is considered from the viewpoint of inhabitation of freeelectron movement by a hyperfine structure, and thereby the structureisometrically transmitting the electric field of light, as shown in FIG.1, is proposed. The inventors have been studied that structure indetail. As a result, it is found that the electrode comprising anelectrode layer composed of a metal part and openings, as a whole, cantransmit all directions of polarized light if the metal part continueslinearly without breaks by the openings in a straight length of not morethan ⅓, preferably not more than ⅕ of the wavelength of incident light.On the other hand, any pair of point-positions in the metal part iscontinuously connected. In other words, the metal part as a wholecontinues although the openings are provided therein, and thereby thefunction as an electrode is ensured while the resistivity is enoughreduced, according to the volume ratio of the openings, to keep arelatively high electro conductivity.

It has been hitherto very difficult to produce completely evenly theabove structure, in which the metal part continues linearly in astraight length of not more than ⅓ of the wavelength of light, on thewhole metal layer. However, the inventors have found that, only if 90%or more, preferably 95% or more of the whole metal layer has thestructure in which the metal part continues linearly in a straightlength of not more than ⅓ of the wavelength of light, the transparencyto light is kept to achieve the object of the present invention.

The above description is based on the assumption that incident lightcomes perpendicularly to the surface of the electrode, but thelight-transmitting metal electrode functions not only to perpendicularlyincident light but also to obliquely incident light. Even in the case ofobliquely incident light, the light also cannot penetrate into the metalalthough the movement of free electrons is geometrically inhibited in alonger apparent distance as compared with that in the case ofperpendicularly incident light. When a metal is irradiated with light,the “skin-depth” is defined as the distance over which the penetratinglight falls to 1/e (wherein “e” is a base of natural logarithms) of itsoriginal strength. The skin depth of obliquely incident light is onlyseveral nanometers. Accordingly, even in the case of obliquely incidentlight, the light-transmitting metal electrode also functions well.

For analyzing and confirming that the above structure is formed in themetal electrode layer, the following method can be adopted. An electronor atomic force micrograph of the observed layer surface is subjected toFourier transform, and the correlation wavelength and the correlationfunction are plotted on X- and Y-axes, respectively. The correlationfunction plotted on Y-axis indicates periodicity of the continuousstructure. That is to say, it indicates how much the structure imaged inthe micrograph contains moieties having the repeating unit of aparticular wavelength. The threshold of the correlation wavelength isdetermined at ⅓ of the wavelength of light, and then the integratedvalue of the correlation function in the range of more than thethreshold and that in the whole wavelength region are calculated andcompared. If the ratio of the former per the latter is 10% or less, itcan be considered that 90% or more of the whole surface of the metalelectrode layer continues linearly in a straight length of not more than⅓ of the wavelength of light.

The second theoretical basis, which concerns straightforwardness oflight maintained by reduction of scattering effects and by avoidance ofdiffraction, is then described below.

The present invention is aimed to reduce scattering effects and therebyto improve the efficiency of straightforwardness of light, and hence itis necessary to treat sizes perturbing the incident light as parameterswhen the surface structure is defined. From this viewpoint, it is foundthat the diameter of the opening is most properly determined by theradius of gyration of the opening structure and thereby that theefficiency of straightforwardness of light can be most properlyrepresented. That is to say, the radius of the opening in the surfacestructure according to the present invention is defined as its radius ofgyration, and accordingly the diameter is double the radius of gyration.Even if the openings have different shapes, the same effect of theinvention can be obtained as long as the radii of gyration are the same.

In the present invention, the radius of gyration of the opening isdefined as follows. On an image of the opening, circular lines at equalintervals are drawn from the edge. In concrete, on a relief imageobtained by atomic force microscopy, circular lines at equal intervalsare drawn from the edge. The thus-obtained lines are image-processed toobtain the center of gravity. The distance from the center of gravity tothe concavity is then determined, and is processed together with themoment to calculate the radius of gyration R. The radius of gyration canbe also obtained by Fourier transform of the electron or atomic forcemicrograph.

The larger the surface structure is, the more light is scattered. Theeffect of light-scattering is in proportion to the square of the size.Accordingly, the average radius of gyration R of the openings ispreferably not more than ⅙ of the wavelength of incident light. That isto say, the average diameter of the openings is preferably not more than⅓ of the wavelength of incident light. If the average radius of gyrationR is larger than that, Rayleigh scattering is liable to occur and thelight immediately loses straightforwardness. The diameter of theopenings is more preferably not more than ⅕, further preferably not morethan 1/10 of the wavelength of light. As long as the above conditionsare satisfied, the shapes of the openings are not particularlyrestricted. Examples of the opening shapes include cylindrical shape,conical shape, triangular pyramidal shape, quadrilateral pyramidalshape, and other columnar or pyramidal shapes. And two or more shapesmay be mixed. Even if the light-transmitting metal electrode accordingto the present invention contains various sizes of openings, the effectof the invention can be obtained. It is, on the contrary, ratherpreferred that the openings have various sizes because the metal partwith those openings is apt to continue linearly in a relatively shortstraight length. In the case like the above, where the openings havevarious sizes, the diameters of the openings can be represented by theaverage.

Diffraction of light incident on the light-transmitting metal electrodeaccording to the present invention is then described below. When lightpenetrates from the transparent substrate-side to the metal layer-side,the scalar theory tells us as follows. On the assumption that the metallayer-side is regarded as an air layer and that the electrode isregarded as a linear diffraction grating, the condition for causingdiffraction is expressed by the following formula (2):sin θ_(m) −n×sin θ₁ =m×λ/d  (2)whereinθ_(m) is an emission angle, θ₁ is an incident angle on the transparentsubstrate-side, λ is a wavelength of incident light, d is an interval ofdiffraction grating, m is a diffraction order of integer (m=0, ±1, ±2 .. . ), and n is a refractive index of the transparent substrate.Accordingly, the condition for not causing diffraction is that theformula (2) does not have a solution when the diffraction order is theminimum: m=−1. This means that the condition is λ/n<1, and hence theratio of the wavelength of light per the refractive index of thetransparent substrate is the threshold. The refractive index oftransparent substrate used generally is not more than 2.0, and thereforethe diffraction can be avoided if the openings in the present inventionare arranged at such intervals that pitches between the centers of theopenings are not more than ½ of the wavelength of incident light.

Preferably, the relative positions of the openings are arranged atrandom, namely, isometrically. According to the first theoretical basis,the reason for this can be explained as follows. For example, if theopenings are arranged in hexagonal symmetry, the metal part has areasperiodically continued in tri-axial directions and therefore it isthought that the openings cannot inhibit the movement of free electronsisometrically.

For analyzing and confirming whether the openings are arrangedisometrically or not, the following method can be adopted. For example,an electron or atomic force micrograph of the metal layer having theopenings is subjected to two-dimensional Fourier transform to obtain,what is called, a “reciprocal space image”. If the relative arrangementof the openings has periodicity, there are clear spots corresponding tothe packing arrangement in the reciprocal space image. In contrast, ifthe openings are relatively positioned isometrically in completelyrandom arrangement, a ring shape is observed instead of the spots.

In the case where EB (electron beam) lithography system or exposuresystem is used to fabricate the openings, it is easy to produce along-periodical structure having regular arrangement but it is difficultto produce a structure having the openings arranged randomly. On theother hand, in the present invention, a phase-separated form ofblock-copolymer is used as a template. The block-copolymer in thephase-separated form gives openings relatively positioned isometricallyin random arrangement, and hence is suitable for manufacturing thelight-transmitting metal electrode according to the present invention.

The following description is based on the result that a metal electrodehaving fine openings was fabricated and measured in practice. Theelectrode must comprise the metal part which continues linearly withoutbreaks by the openings in a straight length of not more than ⅓ of thewavelength of light. However, from the viewpoint of microfabrication,the openings preferably have an average diameter of 10 nm or more. Ifthe average diameter is smaller than that, it is often difficult tofabricate a light-transmitting metal electrode excellent intransparency.

FIG. 2 is an electron micrograph showing a top surface of thelight-transmitting metal electrode comprising openings according to onepractical embodiment. The light-transmitting metal electrode comprisingthese openings was formed by aluminum vapor-deposition and lift-offprocess in which a diblock-copolymer membrane was used as a template ofthe openings. The process according to the present invention can give alarge pattern having openings of 100 nm or less, which cannot beobtained yet according to conventional photo or electron beamlithographic processes. Needless to say, if the photo or electron beamlithographic processes are improved to produce the similar structure inthe future, it can have the same function as the light-transmissionmetal electrode according to the present invention.

In the present embodiment, diblock copolymer mainly comprising aromaticpolymer and acryl polymer in combination was used. As described below,however, the combination is not restricted as long as one component ofthe diblock copolymer can be selectively removed. Further, the method inwhich nanoparticles are also used as the template (JP-2005-279807(KOKAI)), the in-printing process in which polymer or metal having fineconvexes and concaves is used as a stamp to transfer a relief image, andan EB (electron beam) lithographic system can be also adopted.

The reason why the diblock copolymer of aromatic polymer and acrylpolymer in combination was used in the present embodiment is that thesetwo polymers have large difference in reactive ion etching (hereinafter,referred to as RIE) rate. The theoretical basis thereof is disclosed inU.S. Pat. No. 6,565,763. Examples of the aromatic polymer includepolystyrene, polyvinyl naphthalene, polyhydroxystyrene, and derivativesthereof. Examples of the acryl polymer include polyalkylmethacrylatessuch as polymethylmethacrylate, polybutylmethacrylate andpolyhexylmethacrylate; polyphenylmethacrylate,polycyclohexylmethacrylate and derivatives thereof. Instead of thesemethacrylates, acrylates can be used to obtain the same effects. Amongthe above, diblock polymer of polystyrene and polymethylmethacrylate ispreferred because it can be easily synthesized and it is easy to controlthe molecular weight of each component polymer.

In order to use the diblock copolymer as a template in the presentinvention, it is necessary that blocks in the copolymer formsufficiently self-assembled dot-shaped domains in nano-scale.Accordingly, from the morphological viewpoint, it is most preferred thatthe block copolymer it bulk have dot-structures.

The self-assembled block copolymer is by no means automatically alignedin the desired arrangement. Patterns in a short range are aligned in thesame direction to form grains. The block copolymer is subjected tothermal annealing at a temperature higher than the glass transitionpoint of the block polymer, and thereby the gains grow gradually. It isalready reported by the past study (C. Harrison, et. al., PhysicalReview E, 66, 011706 (2002)) that the growing rate is in proportion tothe ¼ power of the growing time. This means that annealing for onlyseveral hours is enough for the aligned grains to grow to be severalmicrons long.

The inventors have found a methods and conditions to obtain ablock-copolymer in the phase-separated form of a dot pattern having aperiod of 50 to 70 nm. The aligned dot pattern is transferred to thesubstrate by the process described below. A metal electrode is depositedonto the transferred structure, and then the area of the transferredpattern is removed to obtain a light-transmitting metal electrode.

In the present invention, metals constituting the electrode aredesirably selected. Here, the term “metals” means materials which areconductors as simple substances, which exhibit metallic gloss, whichhave malleability, which are in the form of solid at room temperatureand which consist of metal elements, or alloys thereof. In one practicalembodiment, the material constituting the electrode preferably has aplasma frequency higher than the frequency ω of incident light. Inaddition, it is also preferred to have no absorption in the wavelengthregion of light to use. Examples of the preferred materials satisfyingthose conditions include aluminum, silver, platinum, nickel, cobalt,gold, copper, rhodium, palladium, and chromium. Among those, aluminum,silver, platinum, nickel and cobalt are more preferred. However, themetal material is not restricted by those examples as long as it has aplasma frequency higher than the frequency of incident light. Asdescribed above, the present invention is advantageous in that it isunnecessary to use a rare metal such as indium and in that typicalmetals can be employed.

In the present invention, it is necessary to produce a pattern in higherresolution than the limiting resolution of generally used lithography.To produce the pattern of very high resolution, it is preferred to adopta lithographic process using a block polymer as an etching mask.

One example of the above process is explained below with FIG. 3 referredto.

First, a transparent substrate 1 is prepared. If necessary, an organicpolymer layer 5 is coated thereon in a thickness of 50 to 150 nm. Theorganic polymer layer 5 is preferably provided so as to improve theaspect ratio of mask pattern in etching the substrate.

Secondly, on the organic polymer layer, an inorganic substance layer 6is coated or deposited in a thickness of 5 to 30 nm. The inorganicsubstance layer 6 functions as an etching mask when the underlyingorganic polymer layer 5 is subjected to oxygen plasma etching. Theorganic polymer layer 5 is easily engraved by the oxygen plasma etching,but the inorganic substance layer 6 can have strong resistance againstthe oxygen plasma etching if proper inorganic substance is selected asthe material thereof.

Finally, a block-copolymer membrane 7 is spin-coated on the inorganiclayer to obtain a composition to be etched. A diblock copolymer isspin-coated, and then is subjected to thermal annealing on a hot plateor in an oven to form dot-shaped microdomains 8 (FIG. 3 (A)). After theblock copolymer is aligned, if one polymer component is easily removedby etching, the nanoscale domains 8 of the aligned and remaining otherpolymer component can be used as an etching mask. In view of this, adiblock copolymer of aromatic polymer and acryl polymer in combinationis preferred since these two blocks give large etching contrast. Forexample, polystyrene and polymethylmethacrylate have very different RIErates, and hence domains of aligned polystyrene can be selectively madeto remain to use as an etching mask for following process.

After one phase of the block copolymer is selectively removed to form adot pattern, the underlying layers are subjected to etching by using thedot pattern as a mask. However, typical polymers constituting the blockcopolymer have insufficient durability to resist the etching applied tothe hard substrate. To overcome this problem, the present embodimentadopts a pattern transfer method using the inorganic substance layer 6so that the pattern can have enough aspect ratio to work as a mask. Inthe etching process, gases are so properly selected that the inorganicsubstance and the organic substance including the polymer can have verydifferent etching rates. In the present embodiment, an etching processof RIE with oxygen gas was carried out. The inorganic substance layerwas not etched by oxygen plasma, and hence the etching contrast betweenthe inorganic substance layer and the underlying organic polymer layer 5was very large. As a result, the organic polymer layer 5 was rapidlyengraved to obtain a dot pattern of high aspect ratio (FIG. 3 (C)).

After the dot pattern is transferred to the organic polymer layer 5(FIG. 3 (C)), the metal electrode layer 9 is provided (FIG. 3 (D)). Forforming the metal electrode layer 9, a metal can be accumulated by, forexample, a vapor-deposition process. As described above, in order to useas a material for the light-transmitting metal electrode, the metal isprefer required to have a plasma frequency higher than the frequency oflight to transmit. The material for the light-transmitting metalelectrode often contains impurities such as oxygen, nitrogen and water.Even in that case, if having a plasma frequency higher than thefrequency of light, the material can transmit the light. After the metalis accumulated, the polymer is removed to obtain the light-transmittingmetal electrode according to one embodiment of the present invention, asshown in FIG. 3 (E).

Materials employable in the present invention are described below indetail.

Examples of the materials for the transparent substrate includeamorphous quartz (SiO₂), Pyrex glass, fused silica, artificial fluorite,soda glass, potassium glass, and tungsten glass. The organic polymer isused for a mask pattern when the metal electrode layer is deposited onthe substrate. It is, therefore, preferred that the polymer can beeasily removed by liquid remover, ultrasonic treatment, ashing, oroxygen plasma. That is to say, the polymer preferably consists oforganic substances only. Examples of the preferred organic polymerinclude polyhydroxylstyrene, novolac resin, polyimide, cycloolefinpolymer, and copolymers thereof.

The inorganic substance layer serves as an etching mask when theunderlying organic polymer layer is subjected to etching, for example,oxygen plasma etching. In consideration of this, examples of thematerials for the inorganic substance layer include deposited silicon,silicon nitride and silicon oxide. Further, spin-coated siloxenepolymer, polysilane end spin-on glass are also advantageous materials ifoxygen plasma etching is adopted.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details and representative embodiments shownand described herein. Accordingly, various modifications may be madewithout departing from the spirit or scope of the general inventiveconcept as defined by the appended claims and their equivalents.

EXAMPLES Example 1

First, a visible light-transmitting metal electrode was produced.

The inventors have found a method to obtain a block-copolymer in thephase-separated form of a dot pattern having a period of 50 to 70 nm.The aligned dot pattern is transferred to the substrate by the processdescribed below. A metal electrode is deposited onto the transferredstructure, and then the area of the transferred structure is removed toobtain a light-transmitting metal electrode. The process is as follows.

A thermosetting resist (THMR IP3250 [trademark], manufactured by TokyoOhka Kogyou Co., Ltd.) was diluted with ethyl lactate by 1:3. Thesolution was spin-coated at 2000 rpm for 30 seconds on a 4-inchamorphous quartz wafer (Photomask Substrate AQ [trademark], manufacturedby Asahi Glass Co., Ltd.), and then heated on a hot-plate at 110° C. for90 seconds, and further heated at 250° C. for 1 hour in anoxidation-free inert oven under nitrogen gas-atmosphere to perform athermosetting reaction. The layer thus formed had a thickness of approx.80 nm.

Spin-on glass (SOG-5500 [trademark], manufactured by Tokyo Ohka KogyouCo., Ltd.) was diluted with ethyl lactate by 1:5. The solution wasspin-coated at 3000 rpm for 30 seconds on the above resist-coatedsubstrate, and then heated on a hot-plate at 110° C. for 90 seconds, andfurther heated at 250° C. for 1 hour in an oxidation-free inert ovenunder nitrogen gas-atmosphere. The layer thus formed had a thickness ofapprox. 20 nm.

Thereafter, 3 wt. % propylene glycol monomethyl ether acetate solutionof polystyrene-polymethylmethacrylate diblock copolymer and 3 wt. %propylene glycol monomethyl ether acetate solution ofpolymethylmethacrylate homopolymer were mixed in a ratio of 6:4. Themixture was filtered through a 0.2 μm mesh to obtain a diblock copolymersolution. The solution was spin-coated at 2000 rpm for 30 seconds on theabove substrate. And then heated on a hot-plate at 110° C. for 90seconds, and further heated at 250° C. for 8 hour in an oxidation-freeinert oven under nitrogen gas-atmosphere to perform phase-separation. Inthe diblock copolymer, the polystyrene moiety had a molecular weight of78000 g/mol and the polymethylmethacrylate moiety had a molecular weightof 170000 g/mol. The thus-obtained copolymer had a morphology in whichmicrodomains of polystyrene in the form of dots having diameters ofapprox. 50 to 70 nm were dispersed in a matrix ofpolymethylmethacrylate. The thus-obtained block copolymer layer had athickness of 50 nm.

The diblock copolymer layer was subjected to etching for 8 seconds underthe conditions of O₂: 30 sccm, 100 mTorr and a RF power of 100 W. Inthis process, the matrix of polymethylmethacrylate in the blockcopolymer was selectively removed, but the polystyrene was not etched.The etching process was carried out under such conditions thatpolymethylmethacrylate among the dots of polystyrene was completelyetched. As a result, the spin-on glass layer in the etched area wascompletely bared. The remaining polystyrene was then used as a maskwhile the spin-on glass layer was subjected to etching of CF₄-RIE for 60seconds under the conditions of CF₄: 30 sccm, 10 mTorr and a RF power of100 W. In this etching process, the spin-on glass layer in the areawhere the matrix of polymethylmethacrylate had been positioned wasselectively etched, so that the dot pattern of polystyrene wastransferred to the spin-on glass layer. Thereafter, the spin-on glasslayer was used as a mask while the underlying thermosetting resist layerwas subjected to etching of O₂-RIE for 90 seconds under the conditionsof O₂: 30 sccm, 10 mTorr and a RF power of 100 W. As a result, columnarstructures having high aspect ratios were formed in the area wherepolystyrene domains had been positioned, to obtain a pattern of columns.

Onto the pattern of columns thus-obtained, aluminum was deposited in athickness of 30 nm by the resistance heat deposition method. And then,Aluminum deposited structure was subjected to aching for 60 sec underthe condition of O₂: 30 sccm, 10 mTorr and a RF power of 100 W. Thepattern was then immersed in water and ultrasonically washed to remove,namely, to lift off the columnar structures. Thus, a light-transmittingssion metal electrode having desired openings was obtained.

The thus-produced light-transmitting metal electrode had openings havingan average diameter of approx. 100 nm, and the openings occupied approx.32% of the whole area. The metal part continuing in 180 nm or lessoccupied 96% of the whole area under the condition that light at 550 nm,which gives the highest luminosity, was used. The transmittance at 550nm was approx. 45%. The resistivity was approx. 30 μΩ·cm.

Finally, distribution of light scattered by the light-transmitting metalelectrode was observed. The produced light-transmitting metal electrodewas cut into pieces of approx. 5 mm×5 mm. A red He—Ne laser beam ofapprox. 638 nm was then applied onto one of the pieces placed 50 cmapart from the laser, and two-dimensional distribution of thetransmitted light was projected onto a screen placed 1 m apart from thepiece. As a result, any clear spot except the center spot was notobserved, and hence it was found that the light was scarcely scattered.

Comparative Example 1

A metal electrode for comparison with Example 1 was produced. Theopenings of the comparative electrode occupied areas in the same ratioas those in Example 1, but the average diameter was approx. 5 μm, whichwas approx. 100 times as large as that in Example 1. In the process forpreparation, a 4-inch amorphous quartz wafer was coated with thephoto-sensitive resist to form a mask having openings of approx. 5 μm,through which the wafer was exposed to light in an exposing system. Theexposed wafer was developed to form a pattern of columns, and thenaluminum was deposited thereon in a thickness of approx. 30 nm.Thereafter, the mask of photo-sensitive resist was removed. Thetransmittance at 550 nm of the obtained metal electrode was measured andfound to be approx. 32%. The resistivity was approx. 30 μΩ·cm.

The distribution of light scattered by the metal electrode produced inComparative Example 1 was observed in the same manner as that inExample 1. As a result, spots in the form of rings were observed at theposition sifted at approx. 5 degrees from the center spot, and hence itwas found that light was scattered more than in Example 1.

Example 2

Secondly, another light-transmitting ssion metal electrode was produced.The openings of the produced electrode occupied areas in a smaller ratiothan those in Example 1

The light-transmitting metal electrode comprising openings in smallerarea can be produced if the block polymer in Example 1 is etched for alonger time. The process for preparation of the light-transmitting metalelectrode having that structure is described below in detail.

A thermosetting resist (THMR IP3250 [trademark], manufactured by TokyoOhka Kogyou Co., Ltd.) was diluted with ethyl lactate by 1:3. Thesolution was spin-coated at 2000 rpm for 30 seconds on a 4-inchamorphous quartz wafer (Photomask Substrate AQ [trademark], manufacturedby Asahi Glass Co., Ltd.), and then heated on a hot-plate at 110° C. for90 seconds, and further heated at 250° C. for 1 hour in anoxidation-free inert oven under nitrogen gas-atmosphere to perform athermosetting reaction. The layer thus formed had a thickness of approx.80 nm.

Spin-on glass (OCD T-7 5500-T [trademark], manufactured by Tokyo OhkaKogyou Co., Ltd.) was diluted with ethyl lactate by 1:5. The solutionwas spin-coated at 3000 rpm for 30 seconds on the above resist-coatedsubstrate, and then heated on a hot-plate at 110° C. for 90 seconds, andfurther heated at 250° C. for 1 hour in an oxidation-free inert ovenunder nitrogen gas-atmosphere. The layer thus formed had a thickness ofapprox. 20 nm.

Thereafter, 3 wt. % propylene glycol monomethyl ether acetate solutionof polystyrene-polymethylmethacrylate diblock copolymer and 3 wt. %propylene glycol monomethyl ether acetate solution ofpolymethylmethacrylate homopolymer were mixed in a ratio of 6:4. Themixture was filtered through a 0.2 μm mesh to obtain a diblock copolymersolution. The solution was spin-coated at 2000 rpm for 30 seconds on theabove substrate. And then heated on a hot-plate at 11.0° C. for 90seconds, and further heated at 250° C. for 8 hour in an oxidation-freeinert oven under nitrogen gas-atmosphere to perform phase-separation. Inthe diblock copolymer, the polystyrene moiety had a molecular weight of54000 g/mol and the polymethylmethacrylate moiety had a molecular weightof 120000 g/mol. The thus-obtained block copolymer layer had a thicknessof 50 nm.

The diblock copolymer layer was subjected to etching for 10 secondsunder the conditions of O₂: 30 sccm, 100 mTorr and a RF power of 100 W.The remaining polystyrene was then used as a mask while the spin-onglass layer was subjected to etching of CF₄-RIE for 60 seconds under theconditions of CF₄: 30 sccm, 10 mTorr and a RF power of 100 W.Thereafter, the spin-on glass layer was used as a mask while thethermosetting resist layer was subjected to etching of O₂-RIE for 90seconds under the conditions of O₂: 30 sccm, 10 mTorr and a RF power of100 W. As a result, columnar structures of high aspect ratios wereformed in the area where polystyrene domains had been positioned, toobtain a pattern of columns.

Onto the pattern of columns thus-obtained, aluminum was deposited in athickness of 30 nm by the resistance heat deposition method. And then,Aluminum deposited structure was subjected to ashing for 60 sec underthe condition of O₂: 30 sccm, 10 mTorr and a RF power of 100 W. Thepattern was then immersed in water and ultrasonically washed to remove,namely, to lift off the columnar structures. Thus, a light-transmissionmetal electrode having desired openings was obtained. Thelight-transmitting metal electrode was observed through the electronmicroscopy, and the result was shown in FIG. 4.

The produced light-transmitting metal electrode had openings having anaverage diameter of approx. 50 nm, and the openings occupied approx. 15%of the whole area. The transmittance of the metal electrode in thevisible region was measured, and the result was shown in FIG. 5. Asshown in FIG. 5, the transmittance was higher than the area ratio of theopenings at any wavelength. The reason why the transmittance graduallyincreased in the shorter wavelength range is presumed to be ascribed tothe property of aluminum, which is that the transmittance becomes higheraccording as the wavelength approaches the plasma frequency. As isevident from FIG. 4, the metal part continued linearly in a very shortstraight length because the openings had irregular shapes. This isthought to be the reason why the transmittance in the whole visiblewavelength region was higher than the area ratio of the openings, whichwas only 15%, as expected from the effect of the present invention. Theresistivity of this light-transmission metal electrode was approx. 50μΩ·cm.

The invention claimed is:
 1. A light-transmitting metal electrodecomprising: a transparent substrate, and a metal electrode layer havinga thickness in the range of not less than 10 nm and not more than 200 nmformed on said substrate, the metal electrode layer comprising: a metalpart for which any pair of point-positions in the part is continuouslyconnected, and a plurality of openings penetrating through said metalelectrode layer, having an average diameter in the range of not lessthan 10 nm and not more than ⅓ of a wavelength of light, the pitchesbetween the centers of said openings being not less than the averagediameter and not more than ½ of said wavelength of light, wherein saidopenings are randomly arranged in said metal electrode layer, 90% ormore of said metal part in said metal electrode layer continues linearlywithout breaks by the openings in a straight length between theadjoining openings of not more than ⅓ of the wavelength of light, andthe resistivity of the metal electrode is equal to or less than 50 μΩcm.2. The electrode according to claim 1, wherein the wavelength of thelight is in the range of 380 nm to 780 nm.
 3. The electrode according toclaim 1, wherein said metal electrode layer exhibits the property thatthe transmittance of light is higher than the area ratio of saidopenings.
 4. The electrode according to claim 1, wherein said metalelectrode layer is made of a metal selected from the group consisting ofaluminum, silver, platinum, nickel and cobalt.
 5. The electrodeaccording to claim 1, wherein the material constituting the metalelectrode has a plasma frequency higher than the frequency of incidentlight.
 6. A light-transmitting metal electrode comprising: a metalelectrode layer having a thickness in the range of not less than 10 nmand not more than 200 nm, the metal electrode layer comprising: a metalpart for which any pair of point-positions in the part is continuouslyconnected, and a plurality of openings penetrating through said metalelectrode layer, having an average diameter in the range of not lessthan 10 nm and not more than ⅓ of a wavelength of light, the pitchesbetween the centers of said openings being not less than the averagediameter and not more than ½ of said wavelength of light, wherein saidopenings are randomly arranged in said metal electrode layer, 90% ormore of said metal part in said metal electrode layer continues linearlywithout breaks by the openings in a straight length between theadjoining openings of not more than ⅓ of the wavelength of light, andthe resistivity of the metal electrode is equal to or less than 50 μΩcm.7. The electrode according to claim 6, wherein the metal electrode isformed on a transparent substrate.
 8. The electrode according to claim6, wherein the wavelength of the light is in the range of 380 nm to 780nm.
 9. The electrode according to claim 6, wherein said metal electrodelayer exhibits the property that the transmittance of light is higherthan the area ratio of said openings.
 10. The electrode according toclaim 6, wherein said metal electrode layer is made of a metal selectedfrom the group consisting of aluminum, silver, platinum, nickel andcobalt.
 11. The electrode according to claim 6, wherein the materialconstituting the metal electrode has a plasma frequency higher than thefrequency of incident light.